tests/core/msg_queue_print: fully initialize msg for sending

On 8-bit and 16-bit platforms `uint32_t` is wider than `void *`, as
pointers are (typically) only 16 bit in size. This causes output like:

    Message queue of thread 2
        size: 8 (avail: 8)
        * 0: sender: 2, type: 0x0000, content: 2701197312 (0)
        * 1: sender: 2, type: 0x0001, content: 2701197313 (0x1)

As seen here, the leading two bytes of `msg.content.value` contain
"random" bits, as those bytes are not explicitly initialized.

This fixes the issue by explicitly initializing the whole `msg_t` via
an initializer list.

Author: maribu

tests/core/msg_queue_print/main.c

@@ -39,8 +39,10 @@ int main(void)
 
     /* fill message queue */
     for (uintptr_t i = 0; i < QUEUE_SIZE; i++) {
-        msg.type = i;
-        msg.content.ptr = (void *) i;
+        msg = (msg_t) {
+            .type = i,
+            .content.ptr = (void *)i,
+        };
         msg_send_to_self(&msg);
     }
 
@@ -55,8 +57,10 @@ int main(void)
 
     /* fill up message queue again */
     for (uintptr_t i = QUEUE_SIZE; i < QUEUE_SIZE + QUEUE_SIZE/2; i++) {
-        msg.type = i;
-        msg.content.ptr = (void *) i;
+        msg = (msg_t) {
+            .type = i,
+            .content.ptr = (void *)i,
+        };
         msg_send_to_self(&msg);
     }